Whenever I try to switch users in Ubuntu 12.04 it goes to a black screen with a cursor and hangs. If I do the ctrl-alt-F6 (or any F1-6 button for that matter) it will switch to a command prompt and then if I switch back to ctrl-alt-F7 I'm presented with the login screen that you get when you return from a screenlock. Is this a bug in Ubuntu or does something need reconfigured?